Optical coherence tomography (OCT) is known as a technique for measuring the intra-lumen tomographic structure of an object having a luminal form, such as a blood vessel. Also, as disclosed in U.S. Pat. No. 6,445,939B (Patent document 1), an optical probe inserted into the lumen of an object for performing such OCT measurement is known. The optical probe has a structure in which the tip (distal end) of a single mode optical fiber is connected to a graded index optical fiber having substantially the same diameter as the single mode optical fiber. By constituting the graded index optical fiber as a lens (GRIN lens) in such manner as the working distance is longer than 1 mm and the spot size is smaller than 100 μm, an object having an inner radius of more than 1 mm can be measured at spatial resolution finer than 100 μm according to the OCT method.
Furthermore, as disclosed in U.S. Pat. No. 6,891,984B (Patent document 2) and U.S. Pat. No. 7,813,609B (Patent document 3), by connecting an optical fiber, a GRIN lens, and a deflecting optical element altogether and by covering them with a transparent tube so that the air may be enclosed adjacent to the oblique end face of the deflecting optical element, it is made possible to make an optical probe which can reflect light in a lateral direction at the oblique end face. By rotating such optical probe, the tomographic structure of a luminal object, such as a blood vessel, can be measured by OCT.